Low voltage protective gaps having magnetic means for circulating arcs



3,230,416 AVING MAGNETIC G ARCS 2 Sheets-Sheet 1 L A T -Hm s T Rum JGw Hmm WMC MMR O SMF .R s Pm SEE MM Jan. 18, 1966 Low vom1 Filed April 10, 1965 Jan. 18, 1966 s, R, SMITH, JR ET AL 3,230,416

LOWNOLTAGE PROTECTIVE GAPS HAVING MAGNETIC MEANS FOR CIRCULATING ARCS Filed April l0, 1963 2 Sheets-Sheet 2 ff/g3 @g4 United States Patent O "P 3,230,416 LOW VOLTAGE PROTECTIVE GAPS HAVING IIAGNETC MEANS FOR CIRCULATING RCS Sidney R. Smith, Jr., Stockbridge, and Eugene C. Sakshaug, Lanesborough, Mass., assignors to General Electric Company, a corporation of New York Filed Apr. 10, 1963, Ser. No. 272,078 12 Claims. (Cl. 315-36) This invention relates to over-voltage protective devices, and, more particularly to improved low voltage protective gaps having magnetic means for circulating arcs.

As is known to those skilled in the voltage protection art, the main purpose of over-voltage protective devices, such as voltage gaps, is to protect the insulation of the circuits on which the gaps are applied. Such gaps take many forms. They may comprise a pair of rods spaced from each other in axial relationship. These rods or electrodes may be pointed, or they may terminate in spherical surfaces. The gaps may take the form of a pair of opposing horns or arc runners, e.g., the wellknown horn gap. All such forms, as well as others have been used to form the gap portions of various types of other protective devices, such as lightning arresters. In any case, the two electrodes forming the gap are spaced at a suitable distance so that the voltage required to spark over between electrodes is less than the withstand voltage of the circuit to be protected.

As the required spark-over level is reduced, it becomes necessary to dimension the gap to have smaller physical spacing between electrodes. In many applications this presents no problem since the current magnitude and duration can be limited by suitable series resistance, as for example non-linear resistance material commonly used in series with the gaps which go to make up lightning arresters. Where the voltage protective level is required to be relatively low, however, the resistance value must be kept lower to avoid a high IR voltage following gap spark over. This, in turn, means that the gap current after spark-over tends to be larger and of longer duration, tending to cause damage to the electrode surfaces and degradation of the gap spark-over characteristics. One approach which has been employed to solve this problem, is to make the gaps in the form of horns and to add an auxiliary gapped coil to produce a magnetic eld which aids in rapidly moving the arc out of the starting point between the horns, or arc runners, once spark-over has occurred. However, for low voltage gaps, series gapped coils are undesirable since the spark-over voltage of the coil gap adds to the main voltage gap, increasing the over-all voltage characteristics.

It is well known to use voltage gap electrodes provided with divergent horns whereby arcs established in the spark-over portion of the gap tend to move out to the ends of the divergent horns due to the magnetic effect produced by the current in the electrodes. However, when the distance between the electrodes in the sparkover portion of the gap is about 30 mils or less the arc tends to stick or remain immobile in the spark-over portion of the gap. This causes melting or fusing of the electrodes. To obtain the desired low voltage spark-over characteristics a spacing of less than 30 mils is required. The spacing could be enlarged by partially evacuating the spark gap. This would increase the cost of the device. It would also introduce new problems because of the ionization time lag characteristics of partially evacuated gaps.

In patent application Serial No. 269,548, for Improved Low Voltage Arc Gaps Provided With Runners for Cir- 3,23%,416 Patented Jan. 18, 196

culating Arcs, led April l, 1963, in the name of Sidney R. Smith, Jr., and assigned to the same assignee as this application, a novel new voltage arc gap is disclosed and claimed, in which a trigger electrode is provided between the main gap electrodes to obtain consistent low voltage spark-over characteristics while still maintaining adequate spacing between the main electrodes to allow the arc to be readily moved out of the spark-over portion of the gap. That application also discloses the use of arc runners adjacent to the gap electrodes for circulating arcs of relatively long duration. The arc runners of that application are specially designed to provide for circulating of the arc around the arc runners. The design of the arc runners in that application depends on the current of the arc to cause the arc to circulate.

It has been found that in some cases, where the current of the arc is of a relatively small magnitude, the force generated by the arc is not strong enough to cause the arc to rotate. In such cases, arcs of long duration cause burning of the arc runners, impairing the over-all efficiency of the arc gap structure. It has been discovered that low current arcs can be circulated about the arc runners by the use of magnetic means in the voltage arc gap structure. The magnetic means provide a force on the arc formed between the arc runners to cause the arc to rotate about the runner, despite the small magnitude of the current of the arc.

It is, therefore, an object of this invention to provide a low voltage gap structure having large surface arc runners, with magnetic means to cause the arc to circulate about such arc runners.

A further object of this invention is to provide a gap structure in which the gap electrodes cause the arc to be moved onto arc-running surfaces, and magnetic means are provided in conjunction with the arc running surfaces to cause the arc to rotate about such surfaces.

In carrying out this invention in one form a low voltage arc gap construction is provided which comprises a pair of gap electrodes having a spark-over portion and diverging outer portions. A trigger electrode is placed between the gap electrodes with a portion extending into the spark-over portion, to enable the sparkover portion to spark over at desirable low voltage levels. According lto this invention, annular arc runners are provided as part of the gap electrodes, the electrodes being formed to enable the arc to be forced out on to such arc runners. Magnetic means are provided to rotate the arc about the arc runners thereby preventing damage to the surfaces of the arc runners. The arc gap structure thereby prevents deterioration of the gap spark-over characteristics inasmuch as the arc which follows gap spark-over is automatically moved out of the spark-over region and caused to rotate at a relatively remote distance from the sparkover region.

The invention which is desired to be protected will be particularly pointed out and distinctly claimed in the claims appended hereto. However, it is believed that this invention and the manner in which its various objects and advantages are obtained, as well as other objects and advantages thereof, will be better understood from the following detailed description of a preferred ernbodirnent thereof, especially when considered in the light of the accompanying drawings, in which:

FIGURE 1 is a sectional view showing a preferred form of low voltage arc gap structure according to this invention;

FIGURE 2 is a sectional View taken on the line 2 2 of FIG. l showing other details of the preferred form of low voltage arc gap structure;

FIGURE 3 is a plan view of an additional arc runner which may be used between the arc runners of FIGURE 1 to split the arc between such arc runners into smaller arcs;

FIGURE 4 is a sectional view of the arc runner shown in FIGURE 3, taken on the line 4-4 of FIGURE 3; and

FIGURE 5 is a partial sectional view of an arc gap structure similar to FIGURE 1 showing the positioning of the additional arc runners of FIGURES 3 and 4.

As earlier noted, in forming a low voltage arc gap it is desirable to maintain adequate spacing between the gap electrodes so that an arc formed therebetween can be readily moved out of the spark-over portion to prevent damage to its spark-over characteristics. It has been found that where the spacing of the spark-over portion of the gap electrodes is small, in the order of 30 mils or less, it is very difficult to move the arc out of the sparkover portion. However, to obtain desired low voltage spark-over characteristics necessary to enable a low voltage to stress the air in the gap to cause an arc to form, it is necessary to have small spacing between the electrodes in the spark-over portion of the gap. As is set forth in application Serial No. 269,548, both requirements are met by using a trigger electrode between the gap electrodes with a portion of the trigger electrode extending into the spark-over portion of the gap. The trigger electrode is electrically connected to one of the gap electrodes by means of an electrical impedance to thereby enable the gap to spark over, first from the electrodes to the trigger electrode, and then between the main electrodes. While not forming a part of this invention, the construction and operation of the low Voltage arc gap using a trigger electrode will be briefly described.

Reference will now be made the drawings, in which like numerals are used to indicate like parts throughout the various views thereof. Considering FIGS. 1 and 2 of the drawings, a low voltage arc gap structure is shown according to a preferred embodiment of this invention. As shown in FIGS. l and 2, the `arc gap structure comprises a pair of terminal members 18 and 12 which are placed at opposite ends of the gap structure. The terminal members and 12 are electrically connected to the gap electrodes 14 and 16 to allow an electrical potential to be applied across the spark-over portion 18 of the gap electrodes 14 and 16. The spacing between electrodes 14 and 16 at the spark-over portion 18 is suiciently large to enable any arc formed in the spark-over portion of the gap to be readily moved out of the sparkover portion 18 and along the enlarged or divergent portions 20 and 22 of the gap electrodes 14 and 16.

In order to provide the desired low voltage breakdown characteristics to the spark-over portion 18, a trigger electrode 24 is provided which is mounted between the electrodes 14 and 16. The trigger electrode 24 is insulated from both electrodes 14 and 16 by insulating members 26 and 28 which may be mica plates or any other desired insulating material. In the preferred embodiment shown, electrically conductive spacer members 30 and 32 are provided, electrically connected to electrodes 14 and 16, respectively. Of course, the spacer members could be formed as part of the electrodes, if desired. The trigger electrode 24 is electrically connected to one of the electrodes, for example, electrode 16 in FIG. l, by means of an electrical impedance indicated at 34 in FIGS. l and 2 shown as connected to spacer 32. The tip 36 of the trigger electrode extends into the spark-over portion 18 between the gap electrodes 14 and 16, as shown. Thus, small trigger gaps 38 and 40 are provided between the trigger electrode 24 and the spark-over part `of each of the gap electrodes 14 and 16.l

no current flowing through the trigger electrode, before arcing, there will be no voltage drop across impedance 34. Therefore, the entire voltage stress is placed across the trigger gap 38 between electrode 14 and electrode 24. 4Thus a small voltage will provide suicient stress to ionize the air and form an arc in the trigger gap 38 between that electrode 14 and trigger electrode 24.

As soon as the arc forrns between gap electrode 14 and the tip 36 of the trigger electrode 24, current will ilow in the trigger electrode through the impedance 3.4. This current ow will change the electrical potential between the trigger electrode 24 and the gap electrode 16. Since there is very little resistance in the trigger gap 38 substantially the entire electrical potential across the terminals 10 and 12 will now be formed across the trigger gap 40 between the tip 36 of trigger electrode 24 and the gap electrode 16. Of course, since an arc has already formed between electrode 14 and trigger electrode 24 ionized air is already available in the trigger gap 38, thereby enabling an arc to readily form between the trigger electrode 24 and the lower arc gap on electrode 16.

As will be understood, as soon as the second arc forms between the trigger electrode 24 and the gap electrode 16 Substantially no current will flow in the impedance 34 inasmuch as there is much less resistance in the trig; ger gap 40 than in the impedance 34. Therefore, the arc will immediately transfer from electrode 14 to electrode 16 since the spark-over portion 18 will have less resistance than two arcs formed across the trigger gaps 38 and 40. This shifting of the arc to a point away from the trigger electrode is also aided by the magnetic ield created by the currents in the electrodes 14 yand 16. Thus it is seen that by means of the trigger electrode 24 the spark-over portion 18 of the electrodes 14 and 16 Will be caused to arc over at low voltage potentials thereby forming the desired low voltage .arc gap.

When the arc forms in the spark-over portion 18 the current ilow through the electrodes 14 and 16 causes a magnetic force which forces the 4are to move out along the enlarged or divergent portions 20 and 22 of the gap electrodes 14 and 16. As will be understood, as the arc is formed in the spark-over portion 18 a magnetic flux4 is generated by the current flow within the arc. The flux, which is generated in the spark-over portion 18 on the side adjacent the trig-ger gap 24, is narrowly conned within a small space, as can be seen from an inspection of FIG. 1. However, the flux which is generated on the opposite side of the arc within the enlarged or divergent portions 20 and 22 of the electrodes 14 `and 16 is provided with a substantial expansion area. That is, the current path through the electrodes 14 and 16 and through the arc, forms a U-shaped configuration. The flux density is greater inside the U, especially at the bight of the U, than it is outside the U. Therefore, as will be understood, the force of the magnetic flux on the arc, which is lat right angles to the flux field will be greater on the side of the arc adjacent the trigger gap 24, that is, at the bight of the U, and will force the arc out along the portions 28 and 22 of the gap electrodes 14 and 16. In this manner it will be seen that once the arc is formed in the spark-over portion 18, the arc is caused to move out on the portions 20 and 22 of the electrodes, thereby preventing the arc from sticking in the spark-over portion and preventing deterioration of the spark-over voltage level of the arc gap structure.

Of course, as will be readily understood, many times it will be necessary for the low Voltage gap structure to be used in an environment which involves either moisture or corroding films. Obviously, in such environments it is desirable that the structure be sealed against the atmosphere. In order to provide the desired sealing of the low voltage gap structure, shown in FIGS. 1 and 2, a casing member 42 is provided substantially surrounding the low voltage gapstructure, as is clearly shown in FIGS. 1 and 2. In FIG. 2 the casing member is shown as an annular member. As will be understood the member 42 may be any shape desired; however, the preferred embodiment is annular for reasons which will hereinafter appear. The upper and lower portions of casing 42 are sealed by insulating members 44 and 46, which may be for example a water resistant :plastic such as phenolic, and have outer circumferential portions which iit under the spun over ends 48 and 511 of the casing member 42, as is shown most clearly in FIG. l of the drawings. If desired, a flexible insulating material, indicated at 52, may be utilized between the spun over members 48 and 50 of the case 42 and the insulating end members 44 and 46 to ensure `a substantially tight seal for the end members. Openings are provided through the end members 44 and 46 for the terminals 10 and 12, as is shown most clearly in FIG. 1. Beneath the end members 44 and 46 gaskets may be provided, indicated at 54 and 56 of the drawing, between the end members 44 and 46 and gap electrodes 14 and 16. The gaskets S4 and 56 are preferably of a flexible insulating material such that when the ends 48 and 50 of casing 42 are spun over the end members 44 and 46 the gasket-s 54 and 56 will be compressed to form a tight seal about the gap electrodes 14 and 16 to prevent entry of .any moisture or corroding fumes into the arc gap structure.

In accordance with the preferred embodiment of this invention, the gap electrodes 14 and 16 are annular members conforming to the shape of the annular casing 42. This configuration of the gap electrodes is best shown in FIG. 2 of the drawing. The gap electrodes are provided with a dished or indented portion in the manner shown in FIG. 1, so as to provide the desired spark-over portion 18 as earlier described. The dishing or indentation of the electrodes also serves to provide for the mounting of the magnetic means which is utilized to circulate the arc about the arc running extremities of the gap electrodes 14 and 16.

In order to ensure the movement of the are formed in the spark-over portion 18 out of such spark-over portion and onto the arc running portions of the electrodes 14 `and 16, which are formed by the enlarged outer portions 20 and 22, the electrodes may be cut or lanced, in the manner shown in FIG. 2 at 58, S9 and 60. As will be readily apparent, by means of the lances or cuts 58,

59 and 6G the current which flows from the terminal 10 to terminal 12, when the spark-over portion 18 has sparked over, will be concentrated in the portion of the electrodes between the cuts, such as 58, 59 .and 60, and thereby provide a stronger magnetic force for moving the arc out of spark-over portion 18 and onto the arc running extremities 20 and 22 of electrodes 14 and 16.

In order to provide the desired circulation of the arc about the arc running surfaces 26 and 22 of the electrodes 14 and 16, magnetic means are provided, in the form of radially polarized ring permanent magnets 62 and 64, in the illustrated preferred embodiment. As shown in FIG. 1 of the drawing, the ring magnet 62 is placed about the dished portion of electrode 14 between its arc running surface 20 and the gasket 54. In a similar manner, a second ring magnet 64 is provided about the dished portion of electrode 16 between its arc running surface 22 and the gasket 56. In order to prevent any excess heat, which may be generated during the operation of the arc gap structure, from affecting the magnets, thermal insulation, indicated at 66, isy provided between the magnets 62 and 64 and the electrodes 14 and 16.

As will be readily understood, with the ring magnets being permanently radially polarized a magnetic flux is generated owing from the outer diameter to the inner di- Y ameter, or the inner diameter tothe outer diameter, de-

' between the arc running extremities 21) and 22 of the gap electrodes 14 and 16. With an arc formed between the arc running surfaces 20 and 22, and applying the wellknown lefthand motor rule, the force on the arc due to the magnetic flux of ring magnets 62 and 64 will be normal to the direction of the current flow and also normal to the direction of the tiux field, thereby causing the arc to rotate about the arc running surfaces 20 and 22 in either a clockwise or a counterclockwise direction. For example, consider the ring magnet 62 as vbeing radially polarized, with the north pole on the outer diameter of the magnet and the south pole on the inner diameter of the magnet. Obviously, the polarization of the ring magnet 64 would be in the same direction so that the flux fields would combine within the arcing area of the arc running portions lof the electrodes 14 and 16. With the polarization as described, the flux ield will ow from the north to the south pole; that is, from left to right, through the left-hand side of the arc gap structure shown in FIG. l. Considering the current to be flowing from terminal 10 to terminal 12, the current flow in the area formed between the arc runners would be in a direction from top to bottom, as viewed in FIG. l. Applying the left-hand motor rule to this polarization and current flow it is seen that the force of the magnetic field on the arc at the left side of the arc gap structure would be coming out of the plane of the paper of FIG. 1, thus causing the arc to rotate about the arc runners in a counterclockwise direction, as Viewed from FIG. 2. It will be readily apparent from the above discussion that by means of the radially polarized magnets 62 and 64 a constant magnetic ux field will be provided between the arc running surfaces 20 and 22 of the electrodes 14 and 16, thereby causing the arc formed between the arc running surfaces to rotate about the surfaces. As is well understood by those skilled in this art, the force causing the arc to rotate about the arc runners is proportional to the product of the flux field from the magnets 62 and 64 and the current in the arc. When and if the current direction reverses, the direction of the arc rotation also reverses.

It is desirable to have the arc proceed radially outward, from the starting point at spark-over portion 18, to the arc running surfaces 20 and 22, and then rotate at the outer extremities of the arc running surfaces 2t) and 22 of the electrodes 14 and 16. In other words, once spark over occurs, it is desirable to have the arc move out of the spark-over portion 18 quickly, and then rotate around the arc running surfaces 20 and 22, without returning to the starting area. The force tending to move the arc radially outward from the spark-over portion 18 to the outer extremities of the arc running surfaces 20 and 22 is proportional to the radial component of the current in the electrodes feeding the arc, but only that radial component in the area where the arc is at any given moment. This radial component can be increased by providing a series of radial cuts, indicated at 5S, 59

f and 60 in FIG. 2, spaced about the electrodes 14 and 16 in the manner shown in FIG. 2. By means of cuts 59 and the cuts 5S and 60, the current flowing in electrodes 14 and 16 is made more effective in moving the arc radially out to the extremities of the arc running surfaces 20 and 22 of the electrodes 14 and 16 and in keeping the arc at such extremities, radially remote from the spark-over portion 18.

In some instances it will be apparent that it will be desirable to split the arc formed between electrodes 14 and 16 into smaller arcs for purposes of quenching or extinguishing the arc and restoring the dielectric of the are gap structure. In order to provide for splitting of the arc between electrodes 14 and 16, ring members, such as shown in FIGS. 3 and 4, may be utilized. As is shown in FIGS. 3 and 4, a ring member 70 is provided which is in the form of a metallic member, such as, for example, copper. The ring member is provided with a plurality of inwardly extending arcuate notches, as indicated at 72 and 74. As can be seen, the notches 72 and 74 extend arcuately from the inner diameter of the ring member 70 into approximately the center of the ring member between the inner and outer diameter thereof. The notches 72 and 74 are wider at the opening at the inner diameter than at the apex of the notches. As will be understod, when the arc is rotating between the arc runner surfaces of the electrodes 14 and 16, and a ring member such as 70 is placed between electrodes 14 and 16 the arc will tend to form in a U-shape about the inner diameter of the ring member 70. However, as the arc approaches one of the notches 72 or 74 in its rotation about the arc runner surfaces of electrodes 14 and 16, the arc will tend to straighten between the electrodes and travel into one of the notches formed in the ring member 70. The notches '72 are arcuate in one direction, while the notches 74 are arcuate in the opposite direction. This is necessary with the use of the ring magnets since the arc may rotate in either direction about the arc running surfaces and 22 of the electrodes 14 and 16, depending on the direction of current ow between the terminals 10 and 12.

When the arc reaches the apex of one of the slots 72 or having formed a plurality of smaller arcs between the arc runner surfaces of electrodes 14 and 16 and any ring members, such as 70, inserted between the electrodes.

The construction utilizing ring members for splitting the arc between the electrodes 14 and 16, is more clearly shown in FIG. 5 of the drawings. As can be seen from FIG. 5, a pair of arc splitting ring members 70 and 76 are provided between electrodes 14 and 16. The members are shown as being embedded in an insulating member 80 to hold such members in a desired equidistant position between each other and the electrodes 14 and 16. As will be apparent, as the arc rotates between the electrodes 14 and 16 it will enter one of the plurality of notches 72 or 74, in the ring members 70 and 76 and will then be split into three separate arcs, one arc formed between the arc runner surface of electrode 14 and arc ring member 70, a second arc formed between the ring member 70 and the ring member 76 and a third arc formed between the ring member 76 and the arc runner surface of electrode 16. In this manner the arc may be split into smaller arcs thus more readily enabling the quenching of such arcs, for example when the potential passes through current zero, thereby restoring the dielectric value of the arc gap structure.

Of course it will be obvious to those skilled in this art that the use of electrodes such` as 14 and 16, with arc running surfaces and ring magnets, may also find utility in arc gaps of relatively high potential and low current.

. For example, in any arc gap using electrodes with annular arc running surfaces, such as electrodes 14 and 16, it may be desirable to rotate the arc formed between such electrodes. Obviously, with high potential an arc gap structure would not require a third electrode. Clearly, once the arc is formed in the gap structure, the arc running surfaces of electrodes 14 and 16 and the magnetic means 62 and 64 will be useful in rotating the arc in the manner previously described.

From the above it will be apparent that there has been provided an arc gap structure which may be utilized for arcing on low voltages, and which maybe readily able to rotate arcs having a small current and being of substantially long duration. Thus it is seen that the invention hereinbefore described has all the objects and advantages hereinbefore set forth. While there has been shown and described the present preferred embodiment of the novel gap structure with magnetic means for rotating low current arc of this invention, it will be apparent to those skilled in the art that various changes may be made, such as for example in the particular shape o f the device, in the particular shape of the various electrodes, all without departing from the spirit and scope of the invention as dened in the claims appended hereto.

What is claimed as new and what is desired to secure by Letters Patent of the United States is:

1. An arc gap structure comprising a pair of metallic terminals connectable across an electrical potential, a pair of gap electrodes mounted between said terminals, a pair of electrical insulation members mounted between said gap electrodes and electrically insulating said gap electrodes from each other, a trigger electrode mounted between said insulation members and electrically connected to one of said gap electrodes through an electrical impedance, said gap electrodes being shaped to provide a narrow spark-over portion with outer annular portions diverging from said sparkover portion, said annular portions forming arc running surfaces, and permanently polarized magnetic means providing a radially directed magnetic field between said annular arc running surfaces for moving an arc about said surfaces.

2. An arc gap structure as claimed in claim 1 in which ring members having arc running surfaces are provided between said arc running surfaces of said pair of gap electrodes.

3. An arc gap structure as claimed in claim 1 in which radial cuts are provided in said electrodes to concentrate current iiow in an arc between said electrodes.

4. An arc gap structure as claimed in claim 3 in which ring members having arc running surfaces are provided between said arc running surfaces of said pair of gap electrodes.

5. An arc gap structure comprising in combination a casing member having insulated end members, said casing being sealed from the outer atmosphere, a pair of terminal members mounted on the opposite ends of said casing, a pair of gap electrodes mounted within said casing, each gap electrode being electrically connected to one of said terminals, a pair of insulating members mounted within said casing between said pair of gap electrodes, a trigger electrode mounted between said insulating members, said trigger electrode being electrically connected through an electrical impedance to one of said gap electrodes, said gap electrodes being shaped to provide a spark-over portion and diverging outer annular portions, said annular portions providing arc running surfaces, and permanently radially polarized ring magnets mounted between said end members and said arc running surfaces, whereby an arc formed between said arc running surfaces will rotate around said arc running surfaces.

6. An arc gap structure as claimed in claim 5 in which radial cuts are provided in said gap electrodes to concentrate current low in an arc between said gap elec` trodes.

7. An arc gap structure as claimed in claim 6 in which ring members having arc running surfaces are provided between said arc running surfaces of said gap electrodes. 8. An arc gap structure as claimed in claim 7 in which said ring members are provided with a plurality of notches spaced about said ring members. y

9. An arc gap structure comprising, in combination, a pair of metallic terminals connectable across an electrical potential, a pair of spaced electrodes mounted between said terminals forming an arc gap, said electrodes being shaped to provide a narrow spark-over portion with outer annular portions diverging from said spark-over area, said annular portions forming arc running-surfaces, and permanently polarized magnetic means providing a radially directed magnetic ield between said annular arc runners for moving an arc about said arc running surfaces.

10. An arc gap structure as claimed in claim 9 in which said magnetic means comprises a pair of radially polar- References Cited by the Examiner lzellrmAgnmle-p structure as claimed in claim 10 in UNITED STATES PATENTS which said pair of spaced electrodes are provided with a 1477303 12/1923 Aucutt 313-197 X plurality of radial cuts. 5 2,825,008 2/1958 Kalb 315-36 X 12. An arc gap structure as claimed in claim 11 in GEORGE N WESTBY, primary Emmi-en which a plurality of ring members having arc running surfaces are provided between said arc running surfaces ROBERT SEGAL Examiner' of said spaced electrodes. S. D. SCHLOSSER, Assistant Examiner. 

1. AN ARC GAP STRUCTURE COMPRISING A PAIR OF METALLIC TERMINALS CONNECTABLE ACROSS AN ELECTRICAL POTENTIAL, A PAIR OF GAP ELECTRODES MOUNTED BETWEEN SAID TERMINALS, A PAIR OF ELECTRICAL INSULATION MEMBERS MOUNTED BETWEEN SAID GAP ELECTRODES AND ELECTRICALLY INSULATING SAID GAP ELECTRODES FROM EACH OTHER, A TRIGGER ELECTRODE MOUNTED BETWEEN SAID INSULATION MEMBERS AND ELECTRICALLY CONNECTED TO ONE OF SAID GAP ELECTRODES THROUGH AN ELECTRICAL IMPEDANCE, SAID GAP ELECTRODES BEING SHAPED TO PROVIDE A NARROW SPARK-OVER PORTION WITH OUTER ANNULAR POR- 